1. Field of the Invention
The present invention relates to a method of producing phosphoric acid starting from phosphate ores.
2. The Prior Art
The production of phosphoric acid, if synthesis is excluded, takes place today principally by two methods: either by the “dry” method, which consumes a great deal of energy, in high-temperature “ore-coke-silica” furnaces, followed by bubbling through of the P2O5 which is extracted in H2O, or by the “wet” method of attacking with acid, principally sulphuric acid, but also nitric, hydrochloric or perchloric acids, of preconditioned (calcination, grinding, sieving) phosphate ores, with the obtaining of a raw phosphoric acid solution. The Ca salts formed, following the attack acid, are eliminated by the crystallisation of the sulphates (from 80° C. to 110° C.), or nitrates (at a temperature of <−5° C. and low filterability) or by carrying out a liquid/liquid extraction of the phosphoric acid by separating it from the calcium chlorides.
The resulting product of these attacks with acid contains all the impurities already existing in the extracted ore. These impurities then contaminate not only the phosphoric acid produced but also the salts crystallised or separated by extraction. This contamination is such that it makes these salts difficult if not impossible to make use of. There exists at the present time a real problem in the exploitation of the gypsum formed during the sulphuric attack. It is necessary to know that the production of 1 tonne of H3PO4 expressed as P2O5 involves the production or more or less 5 tonnes of gypsum in a form which is difficult to exploit at the present time.
Moreover, these attacks all require expensive preconditioning. A fine grinding of the extracted ore is for example absolutely necessary if it is wished to obtain a correct yield by sulphuric attack.
The majority of these methods involve high-temperature reactions which require expensive cooling. The temperature in the liquid-solid attack reactions are difficult to control and these high-temperature attack environments are very detrimental to the equipment.
Various methods are also known comprising an attack on the ore by phosphoric acid or a mixture of several acids, one of which is phosphoric acid (FR-A-2.343.696, U.S. Pat. No. 3,323,864, FR-1.082.404 and EP-A-0 087 323). These methods all involve, after this attack, precipitation of a calcium salt in the product obtained by means of an acid. In some of these methods the precipitated calcium salt still contains the original impurities in the ore, in others the acid used or one of the acids used is hydrofluoric acid or fluosilicic acid, which are expensive acids, which it is necessary to recycle and which are dangerous to handle given their toxicity.
The object of the present invention is a method of producing phosphoric acid making it possible to exploit the by-products to the maximum possible extent, or in any case to greatly reduce the cost of their elimination, whilst proposing a phosphoric acid yield equivalent to or greater than those provided by the known methods. Another purpose of the method is to produce a phosphoric acid suitable for producing fertilisers and for industrial applications, also with a view to obtaining a purified phosphoric acid.
Advantageously, this method must make it possible to avoid grinding or calcination of the extracted ore before the attack. The method according to the invention will preferably be able to be implemented at a temperature which is easily controllable and under conditions which allow reactions which do not greatly attack the equipment used, and control of the apportioning.